Most vehicle seat belt retractors being manufactured currently are of the emergency locking type in which the reel onto which the belt is wound is locked against rotation automatically in response to a device which senses acceleration or deceleration of the vehicle, most commonly upon a collision or upset. Such retractors, though they are recognized to be generally reliable and effective in improving the safety of the vehicle occupant, have one disadvantage; even though the reel may lock reliably and promptly, some extension of the belt can nonetheless occur due to withdrawal of the belt from several loops which remain wound onto the reel, such loops usually being wound relatively loosely and being unwound from the reel by reason of tightening of the loops under the high force imposed on the belt in a collision. This problem has been recognized, and various ways have been proposed for solving it. Most of the proposed solutions involve one form or another of a belt clamping device in which some sort of movable belt-gripping element tightly engages and grips the belt in response to a high force imposed on the belt. Some types of gripping elements, such as curved gripping shoes or eccentric rollers having knurled or otherwise roughened surfaces, are very prone to allowing the belt to slip, inasmuch as the contact area is relatively small. Clamping bars which force the belt into a narrow slot or otherwise produce a sharp fold in the belt, often in conjunction with clamping the belt against a surface, sometimes produce a belt failure by cutting into the belt. It has also been proposed (see U.S. Pat. No. 4,120,466) to clamp a restraint belt between planar clamping surfaces, but it is open to question whether sufficient frictional surface area with a sufficiently high coefficient of friction can be provided in a device of a practical size which can function reliably over a long useful life.